Normally, an occasional rainstorm will wash dust away. But when it's been dry for an extended period and then it rains, the coating of dust gets wet and heavy, and short-circuits the lines. Power poles can even catch fire. They call it "tracking."

Power crews said they expect to be busy repairing outages once more rain moves in, and some tracking-caused outages have already occurred in Tacoma and on the border of Lake Forest Park and Mountlake Terrace.

City Light said if customers hear buzzing on lines near their homes, it could mean the insulators are burning and power might go out.

Utility companies said the problem is more common in South Texas and Arizona.